target/hexagon/idef-parser: Use gen_tmp for LPCFG

target/hexagon/idef-parser: Use gen_tmp for LPCFG

The GET_USR_FIELD macro initializes the output, so the initial assignment
of zero is discarded.  This is the only use of get_tmp_value outside of
parser-helper.c, so make it static.

Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>